PLANET observations of microlensing event OGLE-1999-BUL-23: Limb-darkening measurement of the source star

We present PLANET observations of OGLE-1999-BUL-23, a binary-lens microlensing event toward the Galactic bulge. PLANET observations in the I and V bands cover the event from just before the first caustic crossing until the end of the event. In particular, a densely sampled second caustic crossing enables us to derive the linear limb-darkening coefficients of the source star: c_v = 0.786^+0.080_-0.078 and c_I = 0.632^+0.047_-0.037. Combined analysis of the light curve and the color-magnitude diagram suggests that the source star is a G/K subgiant in the Galactic bulge (T_eff ≃ 4800 K). The resulting linear limb-darkening coefficient of the source is consistent with theoretical predictions, although it is likely that nonlinearity of the stellar surface brightness profile complicates the interpretation, especially for the I band. The global light curve fit to the data indicates that the event is due to a binary lens of a mass ratio q ≃ 0.39 and a projected separation d ≃ 2.42. The lens/source relative proper motion is (22.8 ± 1.5) km s^-1 kpc^-1, typical of bulge/bulge or bulge/disk events.